1. Field of the Invention
The present invention relates to an image forming apparatus suitably used for electrophotographic apparatuses such as copiers, printers and facsimiles; and to an intermediate transfer member used for the image forming apparatus.
2. Description of the Related Art
In general, the image forming apparatus performs a process including: forming a latent electrostatic image on a charged surface of a photoconductor based on an image obtained by an image information reader; developing the latent electrostatic image with a developing device to form a toner image; electrostatically transferring the toner image onto an intermediate transfer member (primary transfer); transferring again the toner image from the intermediate transfer member to a recording medium (a transfer medium) (secondary transfer); and fixing under heating the toner image with a fixing roller or a fixing belt.
A seamless belt has been used as the above intermediate transfer member. Particularly, in full-color image forming apparatuses of recent years, an intermediate transfer belt system is used, in which developed images of four colors: yellow, magenta, cyan and black, are superposed on an intermediate transfer medium (seamless belt) and then the superposed image is collectively transferred to a transfer medium such as paper.
However, in such an intermediate transfer belt system, four developing devices are used for one photoconductor. Such an intermediate transfer belt system has a disadvantage that printing speed is low.
For a system capable of attaining high speed printing, a four-series tandem system is used in which photoconductors for four colors are arranged in a tandem manner, and each color is continuously transferred on paper.
However, in this four-series tandem system, it is quite difficult to achieve sufficient positional accuracy upon superposing respective images because of changes of conditions of paper due to the working environments, forming images where the color images are not accurately superposed on top of each other.
Thus, recently, an intermediate transfer system has been predominately applied in the four-series tandem system.
Under such circumstances, characteristics required for the intermediate transfer belt have become strict to achieve, such as positional accuracy at high-speed transfer, material thereof, etc., but it is necessary to satisfy those characteristics. Regarding the positional accuracy, it is required to inhibit variation in positional accuracy caused by deformation such as elongation of a belt itself due to continuous use. Regarding the material of the intermediate transfer belt, it is required to be flame retardant, since the intermediate transfer belt occupies a large area of an apparatus and a high voltage is applied thereto for transferring an image.
In order to satisfy these requirements, there have been used a polyimide resin and a polyamideimide resin, which are highly elastic and highly heat resistant, as the material of an intermediate transfer belt.
However, an intermediate transfer belt made of a polyimide resin has high strength and thus high surface hardness. Therefore, in transferring a toner image, a high pressure is applied to the toner layer. As a result, the toner particles are locally aggregated, resulting in that part of the image is not transferred in some cases to form a so-called spot-containing image. Also, such an intermediate transfer belt has poor followability to a photoconductor, paper, etc., which are brought into contact with the intermediate transfer belt at transfer positions. Such poor followability may cause insufficient contact portions (spaces) at the transfer positions, leading to uneven transfer.
In recent years, full-color electrophotographic image formation has increasingly been performed on various types of paper, such as commonly-used smooth paper, highly-smooth papers with slip properties (e.g., coated papers) and rough paper (e.g., recycled paper, embossed paper, Japanese paper and kraft paper). In the full-color electrophotographic image formation, followability to such papers that have various surface conditions is important. Poor followability causes unevenness in image density and color toner following irregularities of paper. Thus, there is a need to provide an intermediate transfer belt having excellent followability to paper having different surface conditions.
In order to solve this problem, various intermediate transfer belts have been proposed which contain a base layer and a relatively flexible layer laminated on the base layer.
However, when the relatively flexible layer is used as a surface layer, the pressure during transfer may be reduced. In addition, although the followability to irregularities of paper is improved, toner particles cannot successfully be separated from the surface layer since the toner releaseability of the surface is poor. As a result, the transfer efficiency is decreased while the followability is improved. Furthermore, such a surface layer is problematically degraded in wear resistance and abrasion resistance.
In order to solve these various problems relating to the intermediate transfer belt, intermediate transfer belts each further containing a protective layer have been provided. The protective layer made of a material having sufficiently high transferability cannot comply with the underlying flexible layer and is unfavorably cracked or peeled off. In view of this, it has been proposed that fine particles are attached onto the surface of the intermediate transfer belt (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 09-230717, 2002-162767, 2004-354716, 2007-328165 and 2009-75154).
For example, there has been proposed that the surface of an intermediate transfer belt is coated with beads having a diameter of 3 μm or smaller (see, for example, JP-A No. 09-230717).
However, in the technique proposed in this patent literature, the particles tend to be exfoliated. Thus, this technique is not sufficient to achieve the durability required for the recent electrophotographic apparatuses.
Also, there has been proposed that a layer is formed on the surface of an intermediate transfer belt from a material having an affinity to hydrophobidized fine particles, where particles having a very small particle diameter are preferably used (see, for example, JP-A Nos. 2002-162767 and 2004-354716).
However, in the technique proposed in these patent literatures, the particle layer is thick and has ununiform areas formed due to aggregation of the particles, causing variation in transferability. Thus, this technique is not sufficient to achieve the formation of high-quality images required for the recent electrophotographic apparatuses.
Moreover, there has been proposed that relatively large particles are partially embedded in the resin to realize satisfactory durability as well as satisfactory transferability (see, for example, JP-A Nos. 2007-328165 and 2009-75154).
However, even in this proposal, the particles are ununiformly present in the layer. This technique is still not sufficient to achieve the formation of high-quality images required for the recent electrophotographic apparatuses.
In any of the techniques disclosed in JP-A Nos. 09-230717, 2002-162767, 2004-354716, 2007-328165 and 2009-75154, silica particles are preferably used. The silica particles are strongly aggregated together to fail to form a uniform particle layer, resulting in that the particles tend to be exfoliated.
In view of this, in order to prevent the particles from being exfoliated, there has been proposed that an adhesion layer, etc. is formed as an underlying layer on the surface of an intermediate transfer belt.
However, in this proposal, the adhesion layer is quite poor in toner releaseability and thus, toner particles adhere to the “exposed adhesion layer” present the gaps between the particles (filming), causing a cleaning failure. Furthermore, such inorganic particles as silica tend to scratch and abrade the surface of an organic photoconductor, which is suitably used as a latent electrostatic image bearing member responsible for image formation, when comes into contact with the organic photoconductor at the transfer position, causing a failure of degrading durability thereof.
Therefore, there have still not been provided intermediate transfer members that achieve the formation of high-quality images required for the recent image forming apparatuses. Thus, at present, keen demand has arisen for such intermediate transfer members.